1. Field of the Invention
The present invention relates to a mass spectrometer, an ion mobility separator, a method of mass spectrometry and a method of ion mobility separation.
2. Discussion of the Prior Art
Radio Frequency (RF) ion guides are commonly used for confining and transporting ions. Conventionally a plurality of electrodes are provided wherein an RF voltage is applied between neighbouring electrodes so that a pseudo-potential well or valley is produced. The pseudo-potential well can be arranged to radially confine ions and may be used to efficiently transport ions by acting as an ion guide.
The RF ion guide is capable of functioning efficiently as an ion guide even at relatively high pressures wherein ions are likely to undergo frequent collisions with residual gas molecules. However, although the collisions with gas molecules may cause the ions to scatter and lose energy, the pseudo-potential well generated by the RF ion guide acts to radially confine the ions within the ion guide. RF ion guides therefore have an advantage over guide wire types of ion guides wherein a DC voltage is applied to a central wire running down the centre of a conducting tube. In such arrangements ions are held in orbit around the central guide wire and if ions undergo many collisions with gas molecules then they will tend to lose energy and will eventually collapse into the central guide wire and hence be lost. It is known to use RF ion guides to transport ions through vacuum chambers held at intermediate pressures (e.g. 0.001-10 mbar). For example, the ion guide may be provided to transmit ions from an atmospheric pressure ion source to a mass analyser in a chamber maintained at a relatively low pressure.
When ions collide with gas molecules they may get scattered and lose kinetic energy. If the ions undergo a large number of collisions, e.g. more than 100 collisions, then the ions will substantially lose all their forward kinetic energy. The ions will therefore possess a mean energy which is substantially equal to that of the surrounding gas molecules. The ions will therefore appear to move randomly within the gas due to continuing random collisions with gas molecules. Accordingly, under some operating conditions, ions being transported through an RF ion guide maintained at an intermediate gas pressure can lose substantially all their forward motion and may remain within the ion guide for a relatively long period of time.
In practice, ions may still continue to move forwards for other reasons. It is normally assumed that ions may continue to move forwards due to the bulk movement of gas forcing the ions through the ion guide. Space charge effects caused by the continual ingress of ions into the ion guide and hence the electrostatic repulsion from ions arriving from behind may also effectively push the ions through the ion guide. However, without these influences the ions can, in effect, come to a substantial standstill within the ion guide and hence not emerge at the exit.
A known means for driving ions through an RF ion guide at intermediate pressures is the use of a constant DC electric field. To ensure the ions emerge, or simply to reduce their transit time, an axial voltage gradient may be applied along the ion guide. For example, the ion guide may comprise a segmented multipole rod set ion guide with a DC potential maintained between successive rod segments. The axial electric field causes the ions to accelerate forwards after each collision with a gas molecule. A weak electric field, in the region of 0.1 to 1 V/cm, is adequate for pressures between 0.001 and 0.01 mbar. At higher pressures higher field strengths may be used.
In the pressure region above 0.001 mbar ions in an axial electric field will attain velocities according to their ion mobility. Ions emitted from a pulsed ion source can thus be arranged to separate according to their ion mobility. Ions from a continuous ion source may be gated into a drift region.